Measurement of waveplate retardation using a photoelastic modulator

An apparatus for measuring retardation induced by a subject optical element that exhibits birefringence, comprising:

• a source of polarized light arranged such that the light propagates along a path from the source;
• a light detector located in the path substantially opposite the source;
• a photoelastic modulator located in the path between the source and detector such that the light propagates through part of the photoelastic modulator, wherein the photoelastic modulator exhibits static birefringence,
• a compensating optical element rotatably mounted between the photoelastic modulator and the subject optical element and adjustable to compensate for the static retardation of the photoelastic modulator;
• and
• the subject optical element being rotatably mounted in the path so that the light propagating through the part of the photoelastic modulator propagates through the subject optical element to be incident on the light detector, wherein the retardation of the subject optical element correlates to the intensity of the light that is incident on the detector, and whereby the detector produces an output signal indicative of the intensity.